hack: IPv6-only kind clusters resolve a stale registry address

On `IP_FAMILY=ipv6`, `hack/create-kind-cluster.sh` writes the registry's IPv6
address into the CoreDNS Corefile as a `hosts` entry, read once from
`docker inspect` at cluster-create time. The script recreates the registry
container whenever its port mapping does not match, and Docker hands the new
container a different address. Every cluster created before that point then
resolves `kind-registry` to an address nothing is listening on.

The symptom is worse than a missing entry: the name still resolves, so image
pulls fail with `connection refused` rather than a lookup error, and a
single-cluster dev loop never sees it. The entry is added by #958.